This invention relates to polymeric composites of rigid rod aromatic heterocyclic polymers and flexible thermoplastic polymers and a method for the preparation of composite fibers and films with rigid rod-like aromatic heterocyclic polymers molecularly dispersed in a continuous flexible coil thermoplastic polymeric matrix.
Recent developments in the synthesis of rigid rod-like and flexible coil aromatic heterocyclic polymers has created the opportunity to develop a new class of polymeric structural materials. Of special interest are polymeric composites composed of these rigid rod and flexible coil polymers. The intent is to reinforce the flexible coil polymers with the rigid rod polymers and to fabricate these composites into fiber and film forms with similar or superior properties to that of conventional chopped fiber composites. U.S. Pat. No. 4,207,407 discloses polymeric composites of rod-like aromatic heterocyclic polymers and coil like aromatic heterocyclic polymers and their method of preparation.
As is well known, the strength and durability of the composite is largely dependent upon the existence of an extensive long lasting load-transferring interface between the reinforcing constituent and the matrix. If this interfacial area is small the strength of the composite would be greatly impaired. This is the case when the reinforcing rigid rod polymers segregate into large aggregates with sizes in the micron range. Then, due to a smaller specific surface area (area per unit mass) a high strength, durable reinforced composite cannot be obtained. The intent is then to eliminate this reinforcing constituent-matrix interfacial problem. A need exists, therefore, for fabricating molecular composites with the reinforcing rigid rod polymers molecularly dispersed in a continuous flexible coil polymer matrix having superior strength, dimensional stability and durability.
Since the reinforcing rigid rod polymer is aromatic heterocyclic polymers, they are not amenable to conventional melt processing. They can only be fabricated into useful articles such as fibers and films through solution processing which requires strong mineral or organic acid solvents. Hence, the solution behavior of the two polymer constituents in a common solvent is of critical importance in determining how efficiently the rigid rod-like polymer molecules can be dispersed in the flexible coil-like polymer matrix when fabricated through solution processing.
Studies of the solution morphology of polymer blends of rigid rod and flexible coil heterocyclic polymers have indicated there exists, for any composition of these polymers, a critical concentration point, C.sub.cr, at ambient conditions. Above a critical concentration, the ternary solution containing rigid rod polymer, flexible coil heterocyclic polymers and solvent segregates into two coexisting phases, one optically anisotropic (liquid crystalline) and the other isotropic. The anisotropic domains are composed primarily of rigid rod polymers, while the isotropic matrix retains almost all the flexible coil heterocyclic polymers. When these polymer blends are processed above their critical concentration point at ambient conditions, a macroscopically phase separated composite system is obtained in which rigid rod polymer aggregates are dispersed in the flexible coil heterocyclic polymer matrix.
From a mechanics of composites point of view, this phase segregation (aggregate formation) behavior in a composite system is a detrimental feature. The aggegates do not have a large enough aspect ratio (ratio of length to diameter) as compared to chopped fibers in a conventional composite system. Additionally, the aggregates offer too small a specific interfacial area between them and the matrix to achieve the high rigidity and strength required for a structural material. In short, the reinforement efficiency of a molecular composite is greater than that of a conventional chopped-fiber composite.